Environmental Influences on Wood Structure and Water Transport in the Model Tree Populus.

摘要

Variation in xylem structure and function has been extensively studied across different species with a wide taxonomic, geographical and ecological coverage. In contrast, our understanding of how xylem of a single species can adjust to different growing conditions remains limited. In this thesis, I studied phenotypic plasticity in xylem traits in hybrid poplar ( Populus trichocarpa x deltoides). Clonally propagated saplings were grown under experimental drought, nitrogen fertilization and shade for >30 days. The hydraulic and anatomical traits of secondary xylem were subsequently examined. Substantial variation in the dimensions of xylem cells and wood density was observed. The changes in xylem structure were paralleled by differences in xylem hydraulic conductivity and vulnerability to drought-induced cavitation. In order to gain insights into the molecular underpinnings of different xylem phenotypes, I conducted a microarray analysis of gene expression in the developing xylem of plants receiving high versus low nitrogen (N) supply. I found 388 genes differentially expressed (fold change +/-1.5, P ≤ 0.05), including a number of genes putatively involved in nitrogen and carbohydrate metabolism and various aspects of xylem cell differentiation. The results of this study provide us with gene candidates potentially affecting xylem hydraulic and structural traits.;Furthermore, the results presented in this thesis enhance our knowledge of the mechanisms underlying xylem vulnerability to drought-induced cavitation. Using scanning and transmission electron microscopy, I studied the structure and chemical composition of pit membranes in saplings grown under shade and control light conditions. I found that pit membranes in shade plants were thinner and showed an increased tendency for pore enlargement during membrane dehydration compared to control plants. This difference in pit membrane structure is consistent with greater xylem vulnerability in shade plants. I furthermore showed that pectic homogalacturonans are not abundant in the intervessel pit membranes of hybrid poplar. In a follow-up study, I corroborated this surprising result. Using immunolabeling and two specialized histological methods, I demonstrated that homogalacturonans and calcium are restricted only to a limited region around the edges of the membrane in four angiosperm species including poplar. This finding has important implications for our understanding of pit membranes.